I don't understand....help

I have a 40D, 430ex, and a 580ex. I have finally figured out how to use the 580ex as the master and the 430ex as the slave. I can't seem t get the flash effect from the 430ex. I placed it about 8" from a red plastic cup, with the 580ex flashing straight up, and took the shot, but the 430ex's flash was hardly seen on the side of the cup. I had the 580ex on -3, and the 430ex on +3. What as I doing wrong. I jus can't see the effect from the 430ex. Thanks in advance...Stephen.

couple days ago i tested briefly master/slave function of two of mine metz 58.
slave fired only when master head was in direct position. bounced wouldnt trigger slave.
havent played with them anymore

1shot4u wrote in post #10622244
I have a 40D, 430ex, and a 580ex. I have finally figured out how to use the 580ex as the master and the 430ex as the slave. I can't seem t get the flash effect from the 430ex. I placed it about 8" from a red plastic cup, with the 580ex flashing straight up, and took the shot, but the 430ex's flash was hardly seen on the side of the cup. I had the 580ex on -3, and the 430ex on +3. What as I doing wrong. I jus can't see the effect from the 430ex. Thanks in advance...Stephen.

Are you shooting in M mode (manually setting ISO, SS and Aperture)?